This invention relates to roof bolting for mines, and more particularly to so-called grouted roof bolts that are secured in a bore in a mine roof by a hardenable grouting material bonding the bolt in the bore for supporting the mine roof, and to machines and methods for installing such bolts.
The roof bolt of this invention involves an improvement over grouted roof bolts of the single-pass type, such as shown for example in U.S. Pat. No. 4,055,051, having a tubular body, a cutting structure closing the tube at the upper end thereof, a head at the lower end thereof, packages of a two-component resin grouting material in the tubular body, and an outlet port in the upper end of the tube for discharge of the resin material. This bolt is installed by positioning the head of the bolt in a chuck or other suitable engagement device of a bolting machine, rotating the bolt by turning the head, applying upward force to the bolt via its head during rotation for drilling the bore, pushing a piston into the tubular body to extrude the resin through the outlet port into the annulus between the bolt and the bore in the mine roof, and holding the upward force on the bolt until the resin sets. While bolts of the single-pass type hold out a significant advantage over other grouted bolts; namely, a single bolt serves the three functions of drilling the bore, directing the resin to the annulus and supporting the roof, such bolts have certain accompanying problems which have apparently limited their wide usage in the mining industry. Indeed, applicant knows of no commercial usage of roof bolts of this type.
Among the problems and drawbacks associated with such roof bolts is the lack of an adequately sized passage in the tubular body to enable flow of air to remove drilling debris and cuttings from the bore. The containment of such cuttings, which may be in the form of dust, generated in drilling is necessary not only for respiratory health reasons for miners, but also to minimize explosion risks, particularly in coal mines, as coal dust is highly explosive. While an air passage is provided in the bolt of U.S. Pat. No. 4,055,051, its cross-sectional area is limited by the fact that in the relatively small volume in the tubular body there must also be room for a quantity of resin sufficient to fill the annulus around the bolt. For example, for a bolt formed of tubular stock of conventional one-eighth inch wall thickness to be secured in a bore with an annular one-eighth inch grouting material thickness between the bolt and the bore for adequate bonding strength, the tubular bolt body must be one and one-half inch diameter to provide sufficient volume for the grouting material and an air passage of at least one-half inch diameter. An air passage of this size is considered to be the minimum that will allow drilling chips and dust to pass. Conventional grouted roof bolts of solid bar stock are of one-half to three-quarters inch diameter. Thus, a mining machine cannot carry as many single-pass bolts as it can conventional bolts. And each single-pass bolt requires considerably more drilling time than does a conventional bolt because of its larger bore. Moreover, manufacturing drill bits of this type, so that the packages of resin are secured in the tubular body free of any folds, bends or kinks along the entire length of the bolt, typically some three feet, which could present an air flow obstruction, likely is difficult, costly and time-consuming.
Another difficulty is that the use of such bolts, in a manner that will gain the full advantage thereof, is limited to only mines having a relatively high roof, for example seven or eight feet and perhaps more. This is due to the fact that at the initiation of drilling with the upper end of the bolt in engagement with the mine roof, there must be sufficient vertical space in the mine to accommodate the bolt itself, the piston and piston rod, and the drill chuck and associated drive mechanism, all stacked on top of each other as shown in FIG. 2 of U.S. Pat. No. 4,055,051. As indicated previously, the bolt is often three feet long, and the piston and rod is of approximately an equal length. Unfortunately, many mines in the United States, and particularly coal mines in the eastern United States where grouted roof bolts are widely used, do not have such high roof mines.
The roof bolting system of this invention also involves an improvement over roof bolts and roof bolting machines of the type shown for example in U.S. Pat. No. 4,398,850 having a two-position roof bolting turret which in a first position rotates and advances a drill bit, withdraws the bit and delivers packages of a two-component resin material into the bore, and which in a second position inserts the roof bolt in the bore, rotates the bolt to mix the resin components and holds the bolt pressed against the mine roof until the resin sets. While such roof bolting machines and the older design three-position turret roof bolting machines are widely used in the mining industry, they are relatively complex and expensive, and the subject of considerable maintenance. More importantly, since labor costs for installing bolts is the most expensive factor in roof bolting, the operation of these machines involves significant periods of non-productive time in the transition from one position of the turret to the next. This lengthens the time required for bolt installation, as compared to that for a single-pass roof bolt. More particularly, in the operation of a two-position turret machine, after the bore has been drilled, non-productive time is expended in withdrawing the drill bit. And after the delivery of the packages of grouting material, non-productive time is expended in indexing the turret to its second position and in inserting the roof bolt in the bore. For three-position turret machines, in which drilling and delivery of grouting material are done at separate turret positions, there is the added delay of indexing the turret from its first to its second position.